To build:

  qmake calc.pro
  make

Current terminal widget:

  * Run Octave in a separate thread.  On unixy, interact through pty.
    On Windows systems, use hidden console and mirror contents in Qt
    widget.

    Octave interpreter sees input directly, same as for normal
    terminal application.  Good, because no change in the way Octave
    accepts and parses input.  Bad because GUI is not in control of
    input and must arrange to communicate across threads.

    Terminal window can (in theory) be used to run other applications
    started from the system command (for example).  This feature works
    well enough for us to use less running as a separate process to
    handle paging output, but it doesn't work for all applications.
    For example, I am not able to run editors like Emacs or vi in the
    GUI terminal window.

    To make less work as the pager on Unixy systems, the Octave must
    fork, call setsid in the child to detach from the controlling
    terminal, and then exec the GUI.

    GUI must deal with threads when communicating with the
    interpreter.

Proposal:

  * GUI terminal widget is in control of input.

    Use callback interface for readline to handle command line
    editing.

    Use push parser to parse lines as they become available.  Push
    parser will return status of parse, either complete or needing
    more input.

    Uses the following readline callbacks to manage input and update
    the display.  We may need more in the future to handle other
    features.

      rl_getc_function
      rl_redisplay_function
      rl_completion_display_matches_hook

      rl_prep_term_function (do nothing)
      rl_deprep_term_function (do nothing)

   Call rl_callback_handler_install to setup readlines
   callback mode.  This sets the initial prompt and provides a pointer
   to a function that readline should call when a complete line of
   input is available.

   Call rl_callback_read_char when a character is available for
   readline to handle.  We currently store the character to read in a
   global value, then rl_get_function returns that character to
   readline.

   Call rl_callback_handler_remove when shutting down.

   To avoid global variables, it would be helpful if we could use
   lambda functions/functors as callbacks instead of plain C function
   pointers.

Questions:

  * With this arrangement, would the interpreter have to run in a
    separtae thread?

  * Example parser currently computes results
    immediately.  How do we deal with parsing multiple expressions and
    statements with this method?

  * Do we need text position markers to keep track of the prompt position
    (beginning of current line) when inserting or clearing text?